Most members of the malva family are great for bees; they have that classic open flower shape that lets the bees get into it and reach the nectaries at the base of the petals. This family (malvaecae) includes malvas moschata and sylvestris, which are both native to the UK, but also laveteras and hollyhocks. Another cousin is the 'marsh mallow', althaea officianalis that has a starchy root from which the original marshmallow confectionar was apparently produced. Most flower from the end of June through to August with the malvas first and the hollyhocks and lavateras a bit later.
The bees are mainly after the nectar but on the way to getting it they often get covered in a generous dose of white pollen too, and so take that with them to the next flower. I find that moschata and hollyhocks seem to be the most attractive to both quantity and variety of bees.
This picture shows a honeybee getting a pollen bath on malva moschata which we sell because we find it to be one of the best.